the present invention relates to a power steering apparatus for controlling a steering force of a vehicle by fluid, and more specifically, to a hydraulic control apparatus for feeding pressurized oil to a power steering device and a hydraulic reaction device in a handle or wheel steering force control apparatus in which when the vehicle runs at a high speed, a hydraulic reaction is imparted to the power steering device to stabilize a steering feeling of the handle or the wheel.
The power steering device is widely known in which a power mechanism is provided in the midst of a steering device of the vehicle to lighten the handle steering force. On the other hand, in the steering operation of the vehicle, it is desirable to obtain a greater power at the time of low speed driving than that at the time of high speed driving. In response to this demand, U.S. Pat. Nos. 4,034,825, 3,744,515 and the like have been proposed to control generated torque of the power steering device in response to the speed of vehicle.
A supply of oil in the handle or wheel steering force control apparatus as described is achieved in a system as shown in FIG. 1. An oil passage from an oil pump 11 is branched into two parts, one being communicated with a main valve 12 for controlling a power cylinder 13, and the other being communicated with a hydraulic control valve device 14. Oil pressure from the hydraulic control valve device 14 is controlled by a controller 16 for receiving a signal from a vehicle speed sensor 15 which detects a speed of the vehicle to provide a signal corresponding to the speed so as to control the hydraulic control valve device 14, and oil pressure is fed to the hydraulic reaction chamber 17. Thereby, the hydraulic reaction is imparted to the input shaft at the time of high speed driving to increase a rigidity in the vicinity of a neutral position of the steering handle or wheel for safety travelling. Reaction pressure Pf applied to the hydraulic reaction chamber 17 is determined by an open area of the valve 12 and an open area of a fixed throttle valve provided on the hydraulic control valve device 14. However, there was a drawback such that since a portion in the vicinity of neutral of the steering handle or wheel is greatest in the open area, the reaction pressure Pf does not elevate and the ridigity of the steering handle or wheel when the vehicle travels straight forward at high speed is not increased. Furthermore, the oil always flows, during travelling, toward the return through the fixed throttle valve of the hydraulic control valve device, and therefore, a large quantity of oil was necessary. Because of this, an oil pump becomes large to not only increase cost but also to bring shortage of supply of oil and as a consequence, delay of response of handling occurs and feeling is deteriorated. In case of a spool system in which the hydraulic control valve device comprises a sub-spool whose position is controlled by a plunger of a solenoid controlled by the vehicle speed sensor 15 and a main spool which is axially slidably fitted in the sub-spool and which is moved by oil pressure, there was a drawback, such that since the main and sub-spools have their axial ends urged and supported by springs, the relative position between the main and sub-spools is displaced due to unevenness of a spring characteristic of said springs to fail to obtain the stabilized hydraulic characteristic for a long period of time.